Alternating current microphone transmitter



Oct." 1, 1935. A. K. WARD r AL 2,015,327 I I ALTERNATING CURRENT MICROPHONE TRANSMITTER Filed March 19. 1952 RfCT/F/[R INVENTORS ALBERT K- WARD PALM M- CKMG BY 2 l m ATTORNEY Patented Oct. '1, 1935 PATENT OFFICE ALTERNATING CURRENT MICROPHONE TRANSMITTER Albert K; Ward, Audubon, and Palmer M. Craig, Camden,,N. J., assignors to Radio Corporation of America, a corporation of Delaware Application March 19, 1932, Serial No. 599,860

6 Claims} (01. 179-1) Our present invention relates to transmitters, and more particularly to an improved system for supplying a direct current operating potential to a microphone transmitter. v

Microphone transmitter arrangements of the prior art, regardless of their use,' employbattery sources for supplying the necessary operating potential for the microphone. This is particularly true in the. case of public address and i centralized radio systems. The disadvantages inherent in the use of such battery sources are too obvious to necessitate enumeration.

Hence, it may be stated that it is one of the prime objects of ounpresent invention to provide an arrangement for supplying direct current operating potential to a microphone transmitter wherein the use of batteries is dispensed with, the arrangement employing a low frequency alternating current supply source to produce the required direct current potential in such a manv ner that any high frequency alternating current ripple whichmay be superimposed on the direct current potential will be of such a high frequency that it will be inaudible.

Another important object of the invention is to provide an alternating current system for supplying a direct current operating potential to double, or single, button, close talking microphones wherein the potential is free from all objectionable ,hum which might be present in a supply system using a low frequency, alternating current supply.

Another object of the invention is toeliminate the use of batteries in direct current microphone 5 panel arrangements, thus particularlyenabling all centralized radio system equipment to'be a1- ternatingrurrent operated, the present invention utilizing a 60 cycle supply source but being designedin such a manner that all elaborate fil- 4 tering systems, that would have to be used to reduce hum on aiksgstems using such a supply source, are eliminated.

A more specific object Bfithis invention is to provide a microphone panel Tmit. adapted to" 45 operate from a low frequency alternating current supply, which consists of a. tube oscillator oper- 'ating at a frequency slightly above audio frequencies, and a full wave rectifier so that the frequency is rectified and supplied to a micro- 9 phone as a direct current-operating potential, the

nant circuit 3, 6 being connected to the negative oscillator frequency being doubled in the rec- ,tifier whereby any ripple in the said direct current potential will be of such a frequency that it will be inaudible.

Still other objects of the invention are to' im- 5 prove generally the efliciency of microphone potential supply arrangement, and especially, to provide a microphone adapted to be supplied with a direct current operating potential from alow frequency alternating current source which is 10 reliable in operation, free from burn, and economically manufactured and installed in a public address or centralized radio system.

, The novel features which we believe to be characteristic of our present invention areset l5 forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the draw- 20 ing in which we have indicated diagrammatically one arrangement for carrying our invention into efiect.

Referring now to the accompanying drawing wherein there is shown in diagrammatic man- 25 ner a potential supply unit for a microphone transmitter, it will be observed that the filament, or cathode; circuit of the tube l is arranged to be connected to a. 110 volt, 60 cycle, alternating current source. The said source may be the 30 usual lighting circuit, and the tube I may be for example, a UX245 type of tube.

A filament transformer 2 is utilized to couple the 'low frequencypower source to the filament circuit of tube I, the primary circuit of the transformer 2 including the on-off switch I The grid circuit of the tube includes a resonant circuit which comprises an inductance coil 3 and a shunt condenser 6, the low potential side of the reso- 4 terminal of the anode potential supply source (not k shown) B. The circuit 3, 6 is resonant to the frequency of oscillations which are to be produced by tube 1 and itsv associated circuits. The secondary of the transformer 2 is shunted I by the usual resistor ill, the electrical midpoint of which is connected to the low potential side of theresonant circuit 3, 6 and includes a bias resistor 8, shunted by a by-pass condenser 9,

which is adapted to self-bias the grid of the oscillator tube I.

The anode circuit of tube l includes a resonant circuit comprising an inductance coil 4 and a shunt condenser 1, the resonant circuit 4, I being arranged in series between the anode and the positive terminal of the anode potential supply source B. The resonant circuits 3, 6 and 4, I are both tuned to the same frequency, the latter being the frequency of the oscillations generated, and the coils 3 and 4 are coupled to each other, and a coil 5 connected across a pair of opposite terminals H ofthe rectifier unit II, as shown. The coil 5 functions as a coupling coil for transferring the oscillator frequency to the rectifier unit H. The rectifier unit is preferably of the type well known to those skilled in the art and designated as a Rectox unit. The function of this unit is to rectify the oscillator frequency and supply direct current to the succeeding microphone. 4

It is not believed necessary to describe in detail the construction of such a unit, except to point out that each of the rectifier elements 20 of the unit consists of a copper disc or plate having cuprous oxide formed on one side thereof. Such an element has relatively high conductivity in one direction and relativelylow conductivity in the opposite direction. It converts alternating currents into direct currents, and it is commonly used where a direct current is needed and an alternating current is alone available. It will be noted that the four elements 20 comprise a full Wave rectifier unit II. It is believed that this description of therectifier unit will be sufficient to enable those skilled in the art, in view of the knowledge possessed by them of such rectifier units, to readily construct the present invention.

The rectifier unit I I is employed to supply direct current to a succeeding double button microphone transmitter I4, 14. The microphone is shown in conventional form, since those skilled in the art are well awareof the construction of such a double button, close talking microphone. A double potentiometer is utilized as a volume control, the adjustable leads l2, I2, being employed to regulate the operating potential taken oif across the resistor l2. A double potentiometer is employed in this circuit since a double button microphone is employed. A lead 2| connects the terminal22 of the rectifier unit H to the carbon granule cell in the usual manner, while a lead 23 connects the opposite terminal 24 of the rectifier unit II to the midpoint of the PIi-r mary of the speech output transformer l3.

It should be noted that the speech output transformer i3 is designed to match the 200 ohm output of the double button microphone to any desirable amplifying system adapted to-be coupled to the secondary coil of the output transformer. While the secondary of the transformer l3 has been designated as adapted for connection to a Speech output device, it is to be clearly understood that such a device may be'the amplifier of a public address system, or the power amplifier connected to the distributing lines of a centralized radio system, or to any other desired type of circuit which is adapted to utilize the speech output of a microphone. Of course, the present invention is not limited to a double button microphone, but maybe readily used in connection with a single button, close talking microphone transmitter.

It is believed that the operation of the aforemarized in the following manner.

' minal, the current fiow andpotential difference frequency slightly above audio frequencies. This 5 frequency is transferred to a rectifying unit, and delivered to the microphone 'as a direct current potential on which any alternating current ripple will be above audio frequencies, and hence not audible. In other words, it is desired that 10 the potential delivered to the microphone be free 1 from all objectionable low frequency alternating current hum which might be present in a poorly filtered alternating current supply. The oscillator frequency will be doubled in the full wave rectifier ll so that any ripple in the direct current potential supply to the microphone is necessarily inaudible. If terminal 22 be assumed to be the positive output terminal of the rectifier unit H, and terminal 24 its negative output terbetween the carbon cell and the upper button may be regulated by adjustment of upper con-, tact l2 along potentiometer I2. Similarly the current fiow and potential difference between the 25 I carbon cell and the lower button may be regulated by adjustment of lower contact I 2' on pctentiometer [2. This arrangement therefore provides a means for balancing or equalizing the currents in the two sections of the microphone 30 1 and also for controlling the volume output of the device.

While welhave indicated and described one arrangement for carrying our invention into effect, it"will be apparent to one skilled in the art 36 that our invention'is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of our invention as set forth in the appended claims.

What we claim is:

1. In combination with a double button microphone having a carbon cell, a source of alternating current energy having a frequency above the audible range, a full wave rectifier having its input terminals connected to said source, a conductive connection between one of the output terminals of said rectifier and said carbon cell and a circuit connecting the other output terminal of said rectifier and said microphone buttons 50 including a resistor and a-separate contact connectedto each of said buttons and adjustable along said resistor whereby the direct current passing through each of said buttons may be independently controlled. 55

2. In combination, a source of current potential, an oscillator circuit coupled thereto comprising an electron discharge tube having its anode and grid circuits coupled, the grid and 60 anode circuits being resonant to a frequency put terminals and said to one of said buttons fox-controlling the output volume of said microphone, a conductive connection between one at the rectifier output terhaving a pair of output terminals, means for applying alternating current of superaudible frequency to said rectifier, a circuit including the primary winding of an output transformer and a resistor in series, a lead connecting said microphone button to said resistor, a direct conductive connection between one of the rectifier outcarbon cell and a conductive connection between the other rectifier output terminal and the midpoint of said transformer primary winding.

4. The combination defined in the preceding claim in which the connection between the microphone button and resistor is provided with a contact adjustable along the resistor in order to control the current passing through said microphone.

5. In combination with a microphone having two buttons and a common terminal, a rectifier having a pair of output terminals, means for applying high frequency, alternating current to said rectifier, an output circuit for said'microphone including an inductance coil and a resistor' connected in series, a pair of contacts adjustable along said resistor and each connected minals and the commo and a conductive connection between the other rectifier output terminal and the midpoint of said inductance coil.

6. In combination with a microphone having at least one button and a carbon cell, a vacuum feedback connection between said circuits, means for tuning said grid circuit to a frequency above audibility, a full wave copper oxide rectifier having input and output terminals, means coupling the plate circuit of said oscillator to said rectifier input terminals, an output circuit for said microphone including an inductance coil and a resistor connected in series, a conductive connec tion between said carbon cell and an output terminal of said rectifier and a conductive connection between the midpoint of said inductance coil and the other of said rectifier output terminals.

ALBERT K. WARD.

PALMER M. CRAIG. 25

n microphone terminal tube oscillator having plate and grid circuits, a 10 

